Developmental effects are among the most important and least well-understood concerns in toxicology. Our long-term goal is to identify and understand developmental effects of chemicals that are abundant at Superfund sites, and molecular participants in effects. Polychlorinated biphenyls (F'CBs) are persistent and prevalent contaminants in the global environment, of continuing concern at many Superfund sites, including New Bedford Harbor (NBH). Non-ortho (dioxin-like) PCBs bind to the aryl hydrocarbon receptor (AHR), induce cytochrome P4501 (CYPI) genes, and cause developmental abnormalities. Ortho-substituted PCBs are far more abundant but effects, especially on development, are poorly known. There is no knowledge of which congeners interact with receptors (other than AHR), induce CYPs or exert toxicity in premier nonmammalian models for developmental and environmental toxicology (zebrafish, Danio rerio and killifish, Fundulus heterociitus). Aim 1. This Aim will address the hypothesis that ortho-PCBs elicit distinct molecular and phenotypic responses in zebrafish. Ortho-PCB congeners abundant at the Superfund site at NBH will be studied. We will identify transcriptomic effects with a unique microarray (with probes for all 94 zebrafish CYP genes) and by microfluidic qPCR. Ortho-PCB action as agonists or antagonists of PXR will be detennined. Transcriptomic changes and potential mechanisms will be anchored to phenotypic changes (morphological and behavioral). Aim 2. Ortho-PCBs will be examined as substrates/ligands for CYPs and receptors (likely PXR) implicated in Aim 1. Involvement in toxicity will be assessed by knockdown of CYP or receptor expression. Aim 3. We will determine effects of defined mixtures of ortho-PCBs with non-ortho-PCB and polycyclic aromatic hydrocarbon (PAH) AHR agonists, using a Generalized Concentration Addition model. Aim 4. Studies in killifish will address the environmental and general relevance of findings in zebrafish, and will test the hypothesis that a population that has developed resistance to dioxin-like PCBs is resistant also to effects of ortho-PCBs, expanding our understanding of resistance adaptation. This will be a uniquely comprehensive study of ortho-PCB congener effects on development, and the possible mechanisms.